feat(agents): implement dynamic Sisyphus prompt system with agent metadata

Introduce a new dynamic prompt generation system for Sisyphus orchestrator
that leverages agent metadata for intelligent delegation. This revives the
dynamic-sisyphus-agent-prompt branch with comprehensive refactoring.

Changes:
- Add AgentPromptMetadata, AgentCategory, AgentCost, DelegationTrigger types
- Create sisyphus-prompt-builder with dynamic prompt generation logic
- Add AGENT_PROMPT_METADATA exports to all agent modules (oracle, librarian,
  explore, frontend-ui-ux-engineer, document-writer, multimodal-looker)
- Refactor sisyphus.ts to use buildDynamicSisyphusPrompt()
- Add AvailableAgent type export for type safety

This enables Sisyphus to make intelligent agent selection decisions based on
agent capabilities, costs, and delegation triggers, improving orchestration
efficiency.

🤖 Generated with assistance of OhMyOpenCode
(https://github.com/code-yeongyu/oh-my-opencode)

src/agents/librarian.ts

@@ -1,7 +1,25 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
+import type { AgentPromptMetadata } from "./types"
 
 const DEFAULT_MODEL = "anthropic/claude-sonnet-4-5"
 
+export const LIBRARIAN_PROMPT_METADATA: AgentPromptMetadata = {
+  category: "exploration",
+  cost: "CHEAP",
+  promptAlias: "Librarian",
+  keyTrigger: "External library/source mentioned → fire `librarian` background",
+  triggers: [
+    { domain: "Librarian", trigger: "Unfamiliar packages / libraries, struggles at weird behaviour (to find existing implementation of opensource)" },
+  ],
+  useWhen: [
+    "How do I use [library]?",
+    "What's the best practice for [framework feature]?",
+    "Why does [external dependency] behave this way?",
+    "Find examples of [library] usage",
+    "Working with unfamiliar npm/pip/cargo packages",
+  ],
+}
+
 export function createLibrarianAgent(model: string = DEFAULT_MODEL): AgentConfig {
   return {
     description: